Many electrically-powered devices rely on rechargeable batteries as a source of operating power. Such devices include power tools, trimmers, mowers, chainsaws, vacuum cleaners, and lanterns, as well as electronic devices such as computers and telephones. Rechargeable batteries allow greater portability, but are limited by finite power resources. Therefore, users must be cognizant of the level of remaining battery resources, and plan for eventual depletion.
Upon total depletion of a rechargeable battery, it must be recharged. Recharging can be inconvenient, however, particularly when the user is in the field. For example, the user may not be near a source of ac power to carry out the recharging operation. Also, even if ac power is available, the time required for recharging results in work stoppage.
Rapid recharging systems typically require costly high-power electronics for the delivery of high levels of charging current, along with current limit and overvoltage circuitry for preventing over-charging and resulting damage to the working battery. Slower recharging systems are less costly, but prolong the recharging operation, undermining the basic objective of a quick return to service.
Many users simply carry a spare battery. This practice is common among portable computer and telephone users, and is often the case among users of power tools. Unfortunately, a spare battery can be expensive, and adds to the bulk of equipment that the user must carry in the field. In particular, a spare battery must be provided for each battery used in the field, i.e., for each of several devices. Also, many devices are not designed for spare battery replacement. Further, the user ultimately is limited by depletion of the spare battery.